1. Field of the Invention
The present invention relates to a variable pitch stator vane arrangement for an axial flow compressor, particularly an axial flow compressor for gas turbine engines.
2. Background Information
Generally in prior art axial flow compressors with several stages of variable pitch stator vanes, at the designed operating condition of the compressor all the stages of variable stator vanes are operating at maximum efficiency and each stage of stator vanes has a surge margin. However, when the compressor is operating at conditions in which the rotational speed of the compressor rotor is lower than the designed rotational speed of the compressor rotor it is necessary to vary the angles of the stator vanes to prevent surge or stall of the compressor. It has been found that in high pressure ratio compressors i.e. pressure ratios of the order of 12 to 1 or more, that the variation of the angles of the stator vanes in the presently accepted manner has tended to make any surge or stall worse.
The presently accepted method of varying the angles of the stator vanes uses a proportional method in which the variable stator vanes in each stage are moved a proportion or fraction of their full designed angular displacement. The variable stator vanes in each stage are all moved through the same proportion of their full designed angular movement in unison.
Recent advances in axial flow compressors have brought about the requirement for methods of varying the angles of the stator vanes in a non-proportional method in which the variable stator vanes in each stage are moved a proportion, or fraction, of their full designed angular displacement, but the variable stator vanes in each stage are moved independently of the other variable stator vanes in the other stages. This method has overcome the problem of surge or stall in axial flow compressors of high pressure ratio when operating at conditions of low rotational speeds of the compressor rotor.
A third method of varying the angles of the stator vanes is to move the stator vanes in a non-proportional method in which the variable stator vanes in each stage are moved a proportion, or fraction, of their full designed angular displacement, but the variable stator vanes in each stage are all moved through different proportions of their designed angular movement in unison.